Phosphorus Trichloride

PCl₃ inorganic

Molar Mass

137.333 g/mol

Properties

State	Liquid (colorless, fuming, with pungent odor)
Color	Colorless
Solubility	Reacts with water (hydrolysis to phosphorous acid and HCl); miscible with benzene, ether, carbon disulfide
Melting Point	-112°C
Boiling Point	76°C

About Phosphorus Trichloride

Phosphorus trichloride is a colorless fuming liquid (boiling point 76 °C) with a pungent, eye-watering smell from constant trace HCl emission as ambient moisture hydrolyzes the surface. Its trigonal pyramidal geometry — Cl-P-Cl bond angles around 100° with one lone pair on phosphorus — makes it isostructural with ammonia, but the electron-withdrawing chlorines collapse the basicity to the point where PCl3 is essentially non-coordinating toward Brønsted acids. About 320,000 tonnes per year flow out of three industrial routes (P4 + 6 Cl2 → 4 PCl3 being the dominant one), and almost all of that volume goes downstream into organophosphorus chemistry: phosphite esters as polymer antioxidants and lubricant additives, glyphosate herbicide intermediates, organophosphate pesticides (chlorpyrifos, malathion, parathion before its phase-out), flame retardants for textiles and electronics, and triphenylphosphine for the Wittig reaction. PCl3 is the front-end reagent for nearly the entire chemistry of the P-C bond. As a chlorinating agent it converts carboxylic acids to acid chlorides (3 RCOOH + PCl3 → 3 RCOCl + H3PO3) — a milder alternative to thionyl chloride when the substrate or product can't tolerate SO2 byproduct, and it doesn't generate HCl gas in the process. The Atherton-Todd reaction uses PCl3-derived dialkyl phosphites to make phosphoramidates and phosphate esters under mild conditions.

Where you'll encounter it

If you've ever bought a non-stick frying pan, sat in a Boeing or Airbus seat with flame-retardant fabric, or sprayed Roundup on a driveway weed, the PCl3 supply chain touches all three products somewhere upstream. Lab synthesis of triphenylphosphine — the most-used phosphine ligand in coupling chemistry and the reagent in every Wittig olefination — runs PCl3 + 3 PhMgBr → PPh3 + 3 MgBrCl, and Aldrich sells about 600 tonnes of PPh3 per year that all started as PCl3. Process safety engineers building glyphosate plants design PCl3 storage tanks with continuous nitrogen blanketing because even traces of moisture in the headspace start corroding the carbon-steel walls. The grimmer side: PCl3 was a DEA List I controlled chemical in the U.S. through the 2000s because of its use in clandestine synthesis of methamphetamine via the lithium/PCl3 reduction route, and bulk shipments still trigger DHS Chemical Facility Anti-Terrorism Standards (CFATS) reporting.

Common Uses

Triphenylphosphine synthesis from PCl3 + PhMgBr for Wittig reagents and Pd catalyst ligands
Glyphosate herbicide and chlorpyrifos pesticide manufacture (≈320,000 tonnes/year globally)
Conversion of carboxylic acids to acid chlorides (RCOOH to RCOCl) without HCl gas evolution
Phosphite ester (P(OR)3) production for polymer antioxidants and engine-oil additives
Atherton-Todd reaction for phosphoramidate and phosphate-ester synthesis

Safety Information

GHS H300 (fatal if swallowed), H310 (fatal in contact with skin), H330 (fatal if inhaled), H314 (causes severe skin burns), EUH014 (reacts violently with water), EUH029 (releases toxic gas in contact with water). OSHA PEL 0.5 mg/m³ (0.2 ppm) TWA; ACGIH TLV 0.2 ppm TWA / 0.5 ppm STEL. Hydrolysis on contact with skin or eye fluid generates HCl and H3PO3 burns within seconds. Reaction with metals can liberate phosphine (PH3), itself acutely lethal. DHS CFATS Appendix A theft/diversion threshold quantity 100 lb. Required handling: enclosed system or fume hood, butyl gloves, full face shield, and dry-chemical-only spill response (water makes it worse).

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

P — Phosphorus Cl — Chlorine

Frequently Asked Questions

What is the molar mass of phosphorus trichloride?

PCl3 has a molar mass of 137.333 g/mol: 1 × 30.974 (P) + 3 × 35.453 (Cl) = 30.974 + 106.359 = 137.333 g/mol. With a density of 1.574 g/mL, neat PCl3 is 11.5 M — useful for back-of-the-envelope scaling of acid-chloride preps where you measure by volume rather than mass.

What is the molecular geometry of PCl3?

Trigonal pyramidal at phosphorus, structurally analogous to NH3. The four electron domains (three P-Cl bonding pairs plus one lone pair on phosphorus) give a tetrahedral electron geometry, but the molecular shape — defined by atom positions — is pyramidal because the lone pair is invisible. Experimental Cl-P-Cl bond angles are about 100°, compressed from the ideal 109.5° tetrahedral angle by greater repulsion from the lone pair than from a bonding pair (Bent's rule).

Why is PCl3 important for pesticide production?

Almost every organophosphate insecticide — chlorpyrifos, malathion, parathion, diazinon — is built on a P-O-C or P-S-C skeleton that originates from PCl3 reacting with alcohols or thiols, often with sulfur incorporation to form the thiophosphoryl P=S group that mimics the natural P=O of acetylcholine. The mode of action is phosphorylation of the active-site serine in acetylcholinesterase, which prevents acetylcholine breakdown and causes runaway nervous-system stimulation in the target insect (and, at higher doses, in humans). The 320,000 tonnes per year of PCl3 production largely tracks global demand for these agrochemicals.